diff options
Diffstat (limited to 'arch/s390')
-rw-r--r-- | arch/s390/boot/kaslr.c | 130 |
1 files changed, 92 insertions, 38 deletions
diff --git a/arch/s390/boot/kaslr.c b/arch/s390/boot/kaslr.c index c8549a0474e1..d844a5ef9089 100644 --- a/arch/s390/boot/kaslr.c +++ b/arch/s390/boot/kaslr.c @@ -90,14 +90,95 @@ static int get_random(unsigned long limit, unsigned long *value) return 0; } +/* + * To randomize kernel base address we have to consider several facts: + * 1. physical online memory might not be continuous and have holes. mem_detect + * info contains list of online memory ranges we should consider. + * 2. we have several memory regions which are occupied and we should not + * overlap and destroy them. Currently safe_addr tells us the border below + * which all those occupied regions are. We are safe to use anything above + * safe_addr. + * 3. the upper limit might apply as well, even if memory above that limit is + * online. Currently those limitations are: + * 3.1. Limit set by "mem=" kernel command line option + * 3.2. memory reserved at the end for kasan initialization. + * 4. kernel base address must be aligned to THREAD_SIZE (kernel stack size). + * Which is required for CONFIG_CHECK_STACK. Currently THREAD_SIZE is 4 pages + * (16 pages when the kernel is built with kasan enabled) + * Assumptions: + * 1. kernel size (including .bss size) and upper memory limit are page aligned. + * 2. mem_detect memory region start is THREAD_SIZE aligned / end is PAGE_SIZE + * aligned (in practice memory configurations granularity on z/VM and LPAR + * is 1mb). + * + * To guarantee uniform distribution of kernel base address among all suitable + * addresses we generate random value just once. For that we need to build a + * continuous range in which every value would be suitable. We can build this + * range by simply counting all suitable addresses (let's call them positions) + * which would be valid as kernel base address. To count positions we iterate + * over online memory ranges. For each range which is big enough for the + * kernel image we count all suitable addresses we can put the kernel image at + * that is + * (end - start - kernel_size) / THREAD_SIZE + 1 + * Two functions count_valid_kernel_positions and position_to_address help + * to count positions in memory range given and then convert position back + * to address. + */ +static unsigned long count_valid_kernel_positions(unsigned long kernel_size, + unsigned long _min, + unsigned long _max) +{ + unsigned long start, end, pos = 0; + int i; + + for_each_mem_detect_block(i, &start, &end) { + if (_min >= end) + continue; + if (start >= _max) + break; + start = max(_min, start); + end = min(_max, end); + if (end - start < kernel_size) + continue; + pos += (end - start - kernel_size) / THREAD_SIZE + 1; + } + + return pos; +} + +static unsigned long position_to_address(unsigned long pos, unsigned long kernel_size, + unsigned long _min, unsigned long _max) +{ + unsigned long start, end; + int i; + + for_each_mem_detect_block(i, &start, &end) { + if (_min >= end) + continue; + if (start >= _max) + break; + start = max(_min, start); + end = min(_max, end); + if (end - start < kernel_size) + continue; + if ((end - start - kernel_size) / THREAD_SIZE + 1 >= pos) + return start + (pos - 1) * THREAD_SIZE; + pos -= (end - start - kernel_size) / THREAD_SIZE + 1; + } + + return 0; +} + unsigned long get_random_base(unsigned long safe_addr) { - unsigned long memory_limit = memory_end_set ? memory_end : 0; - unsigned long base, start, end, kernel_size; - unsigned long block_sum, offset; + unsigned long memory_limit = get_mem_detect_end(); + unsigned long base_pos, max_pos, kernel_size; unsigned long kasan_needs; int i; + if (memory_end_set) + memory_limit = min(memory_limit, memory_end); + if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && INITRD_START && INITRD_SIZE) { if (safe_addr < INITRD_START + INITRD_SIZE) safe_addr = INITRD_START + INITRD_SIZE; @@ -127,44 +208,17 @@ unsigned long get_random_base(unsigned long safe_addr) } kernel_size = vmlinux.image_size + vmlinux.bss_size; - block_sum = 0; - for_each_mem_detect_block(i, &start, &end) { - if (memory_limit) { - if (start >= memory_limit) - break; - if (end > memory_limit) - end = memory_limit; - } - if (end - start < kernel_size) - continue; - block_sum += end - start - kernel_size; - } - if (!block_sum) { + if (safe_addr + kernel_size > memory_limit) + return 0; + + max_pos = count_valid_kernel_positions(kernel_size, safe_addr, memory_limit); + if (!max_pos) { sclp_early_printk("KASLR disabled: not enough memory\n"); return 0; } - if (get_random(block_sum, &base)) + /* we need a value in the range [1, base_pos] inclusive */ + if (get_random(max_pos, &base_pos)) return 0; - if (base < safe_addr) - base = safe_addr; - block_sum = offset = 0; - for_each_mem_detect_block(i, &start, &end) { - if (memory_limit) { - if (start >= memory_limit) - break; - if (end > memory_limit) - end = memory_limit; - } - if (end - start < kernel_size) - continue; - block_sum += end - start - kernel_size; - if (base <= block_sum) { - base = start + base - offset; - base = ALIGN_DOWN(base, THREAD_SIZE); - break; - } - offset = block_sum; - } - return base; + return position_to_address(base_pos + 1, kernel_size, safe_addr, memory_limit); } |